PITTSBURGH -- Until recently, graffiti-resistant coatings were only available in solvent-based formulations that exhibited a glossy finish. Now, however, there is a more environmentally friendly alternative for concrete substrates: two-component waterborne polyurethane coatings. In addition to being more eco-friendly, these ultra-low VOC (volatile organic compound) waterborne polyurethane coatings also achieve an unmet need in the construction market: graffiti resistance with a matte finish.

The development of these graffiti-resistant, two-component waterborne polyurethane coatings, their properties, and tests examining the reasons they exhibit good graffiti resistance, will be explored in detail at PACE 2009. This annual conference and exhibition, sponsored by the Painting and Decorating Contractors of America and the Society for Protective Coatings, takes place Feb. 15 - 18, 2009, in New Orleans. Peter Schmitt, senior technology manager and Kathy Allen, associate scientist, both of Bayer MaterialScience LLC, will present the technical paper, Now You See It Now You Don't: Waterborne Polyurethane Graffiti Resistant Coatings, during a session entitled "Durability Defined: Taking Ultra-High-Performance Coatings to the Next Level."

According to the authors, it has been generally accepted that to obtain a surface that is easily cleaned of graffiti, the coating used needed a surface energy lower than the surface tension of the graffiti paint. The lower energy would make it difficult for paint to adhere to the coating, making the graffiti easily removed. However, the waterborne coatings used in the tests had higher surface energies yet were still easy to clean of graffiti.

Using atomic force microscopy (AFM), the authors determined that surface roughness played a factor in a coating's ability to be easily cleaned; graffiti can adhere well to a surface that has imperfections, which are a common trait in coatings with a matte finish. However, without pores to serve as anchors for graffiti paint to stick, a surface with a roughness between 10 and 300 nm is easily cleaned. Both glossy and matte finishes can be achieved in that range.

The authors state that a coating must have a delicate balance of characteristics in order to achieve a matte finish while remaining graffiti resistant. The balance is realized through the proper reaction rate and coalescence speed of a coating as it changes from discrete particles to a continuous film, but without the alignment of a glossy surface. Because a two-component waterborne polyurethane coating is made up of two distinct types of particles, there is competition between the physical change (coalescence) and the chemical reaction (creating urethane groups).

"We conclude that with waterborne polyurethane coatings, it is possible to combine good cleanability in an environmentally acceptable system," said Schmitt.

About Bayer MaterialScience LLC:

Bayer MaterialScience LLC is one of the leading producers of polymers and high-performance plastics in North America and is part of the global Bayer MaterialScience business with nearly 15,400 employees at 30 sites around the world and 2007 sales of 10.4 billion euros. Business activities are focused on the manufacture of high-tech polymer materials and the development of innovative solutions for products used in many areas of daily life. The main segments served are the automotive, electrical and electronics, construction, medical, and sports and leisure industries. Our inorganic basic chemicals unit produces chlorine and related essential products for the chemicals industry.

About Bayer Corporation:

Bayer Corporation, headquartered in Pittsburgh, is a subsidiary of Bayer AG, an international health care, nutrition and high-tech materials group based in Leverkusen, Germany. In North America, Bayer had 2007 net sales of 8.1 billion euros and employed 16,800 at year end. Bayer's three subgroups, Bayer HealthCare, Bayer CropScience and Bayer MaterialScience, improve people's lives through a broad range of essential products that help diagnose, prevent and treat diseases; protect crops and enhance yields; and advance automobile safety and durability.

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